In the LTE (Long Term Evolution) Release 10 standardized by 3GPP (Third Generation Partnership Project), 8×8 MIMO is defined and 8-layer spatial multiplexing is supported. Thus the base station can transmit data of spatially multiplexing of up to 8 layers (see NPL 1).
FIGS. 4A and 4B illustrate an example of a reference signal mapping specific to a user terminal (UE-specific reference signals) defined by the LTE Release 10. In FIGS. 4A and 4B, the horizontal axis is time and the vertical axis is frequency. In the horizontal axis, one division represents one symbol and in the vertical axis, one division represents one subcarrier. In the example illustrated in FIGS. 4A and 4B, 8 antenna ports of the base station are divided into two groups where group 1 includes antenna ports 7, 8, 11 and 13 and group 2 includes antenna ports 9, 10, 12 and 14.
FIG. 4A illustrates a mapping of reference signals of group 1 (antenna ports 7, 8, 11 and 13) and FIG. 4B illustrates a mapping of reference signals of group 2 (antenna ports 9, 10, 12 and 14). As illustrated in FIGS. 4A and 4B, resource elements to which the reference signals are assigned are different in group 1 and group 2. Here, the resource element is an area of one square in FIGS. 4A and 4B, that is, an area having time of one symbol and frequency of one subcarrier. In FIGS. 4A and 4B, although the positions of symbols to which reference signals are assigned are the same, the positions of subcarriers are different.
As in the case of antenna ports 7, 8, 11 and 13 of group 1, the reference signals mapped to the same resource element are multiplied by the sequence to the normal CP (Cyclic Prefix) of LTE as illustrated in FIG. 5, thus the signals are orthogonal to each other. Therefore, a spatially multiplexed signal can be extracted by directing a null to the other layers in the same group. In the same manner, for the antenna ports 9, 10, 12 and 14 of group 2, a spatially multiplexed signal can be extracted by directing a null to the other layers in the same group.